Light olefins, defined herein as ethylene and propylene, serve as feeds for the production of numerous chemicals. Olefins traditionally have been produced by petroleum cracking, for example, by fluidized catalytic cracking (FCC). Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
In addition to cracking petroleum products, the petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefins. The preferred conversion process is generally referred to as an oxygenate to olefin (OTO) reaction process. Specifically, in an OTO reaction process, an oxygenate contacts a molecular sieve catalyst composition under conditions effective to convert at least a portion of the oxygenate to light olefins. When methanol is the oxygenate, the process is generally referred to as a methanol to olefin (MTO) reaction process. Methanol is a particularly preferred oxygenate for the synthesis of ethylene and/or propylene
In an OTO conversion process carbonaceous material (coke) is deposited on the molecular sieve catalysts used to catalyze the conversion process. Excessive accumulation of these carbonaceous deposits will interfere with the catalyst's ability to promote the reaction. In order to avoid unwanted build-up of coke on molecular sieve catalysts, the OTO and MTO processes incorporate a second step comprising catalyst regeneration. During regeneration, the coke is at least partially removed from the catalyst by combustion with oxygen, which restores the catalytic activity of the catalyst and forms a regenerated catalyst. The regenerated catalyst then may be reused to catalyze the OTO conversion process.
In conventional regeneration vessels, coked catalyst is directed from a reactor to a catalyst regenerator. In a catalyst regenerator, a regeneration medium, usually oxygen, enters the regenerator, and coke is removed from the coked catalyst by combustion with the regeneration medium to form regenerated catalyst and gaseous byproducts. The bulk of the regenerated catalyst from the regenerator is returned to the reactor. The gaseous byproducts are forced out an exhaust outlet oriented in the upper section of the catalyst regenerator.
The combustion of the carbonaceous deposits from molecular sieve catalyst compositions during catalyst regeneration is an exothermic process. The exothermic nature of catalyst regeneration presents a unique problem in OTO regeneration systems because the OTO reaction process is operated such that the level of carbonaceous deposits on the molecular sieve catalyst composition is higher than the level typically found on catalyst compositions used in FCC processes. As a result, the amount of heat liberated from the OTO molecular sieve catalyst compositions during catalyst regeneration is significantly greater than the amount of heat liberated from the regeneration of catalysts in FCC processes. The significant amount of heat liberated in regenerating OTO catalyst compositions may exceed the material tolerances of the materials used to form the catalyst regenerator. The heat can also damage the catalyst particles themselves. It is therefore desirable to provide processes and systems for controlling the temperature of a regeneration vessel in an OTO reaction system.